Thermal engine, especially piston engine

ABSTRACT

A thermal engine operating according to the Stirling process, particularly a piston engine which includes at least one hot gas section and one cold gas section between which a gaseous working medium is interchangeable by way of a regenerator. The working medium is heated directly in the hot gas section by the feed and combustion of fuel while the combusted working medium is at least partially replaced at the cold gas section by fresh working medium. An exhaust gas turbo-charger may be utilized for supercharging the fresh working medium. The engine is constructed so that fresh working medium is introduced during the transition from the expansion to compression of the cold gas section with a delay in the introduction of the fresh medium until late in the expansion phase. Either a rotary piston arrangement or an engine with axially reciprocating pistons may be used with both arrangements using piston controlled slot openings for admission of the fresh medium and exhaust of part of the medium. In the axially reciprocating piston arrangement, the piston of the hot gas section is axially aligned in a common housing with the piston of the cold gas section and with the regenerator arranged in the housing between said pistons.

United States atom [19] Pattas [7 5] lnventor: Konstantin Pattas,

Stuttgart-Untertuerkheim, Germany [73] Assignee: Daimler-Benz AG,

Stuttgart-Unterturkheim, Germany [22] Filed: July 24, 1972 21 Appl. No.: 274,513

Related US. Application Data [63] Continuation-impart of Ser. No. 76,789, Sept. 30,

1970, abandoned.

[30] Foreign Application Priority Data Sept. 30, 1969 Germany ..P 19 49 191. 9 [52] US. Cl 1 2318.01, 123/51 BA, 123/191 R, 60/24 [51] Int. Cl. F02b l/00, F02b 23/00, F02b 77/00, F03g 7/06 [58] Field ofSeareh 123/191 R, 5 1 R, 123/51 A, 51 B, 51 BB, 51 BA, 8.01; 60/24 [56] References Cited UNITED STATES PATENTS- l55,087 9/1874 Hirsch 60/24 Primary ExaminerWendell E. Borns Attorney-Paul M. Craig, Jr. et al.

A thermal engine operating according to the Stirling process, particularly a piston engine which includes at least one hot gas section and one cold gas section between which a gaseous working medium is interchangeable by way of a regenerator. The working medium is heated directly in the hot gas section by the .feed and combustion of fuel while the combusted working medium is at least partially replaced at the cold gas section by fresh working medium. An exhaust gas turbo-charger may be utilized for supercharging the fresh working medium. The engine is constructed so that fresh working medium is introduced during the transition from the expansion to compression of the cold gas section with a delay in the introduction of the fresh medium until late in the expansion phase. Either a rotary piston arrangement or an engine with axially reciprocating pistons may be used with both arrangements using piston controlled slotopenings for admission of the fresh medium and exhaust of part of the medium. In the axially reciprocating piston arrangement, the piston of the hot gas section is axially aligned in a common housing with the piston of the cold gas section and with the regenerator arranged in the housing between said pistons.

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1111 3,777,718 1451 Dec. 11,1973

ABSTRACT 22 Claims, 5 Drawing Figures PMENTEUUEII I I I975 SHEET 10F 2 G I F 53 NEGATIVE WORK EXPERIMENTAL ENGINE-0F INVENTION [/2 25 EXTRA NEGATIVE WORK FOR ENGINE NOT USING ENERGY 0F COLD GAS SECTION 3511822 I w mww MAX amine PATENTEB DEC 1 1 I975 SHEET 2 UF 2 THERMAL ENGINE, ESPEUlAlLLY PISTON ENGINE This application is a continuation-in-part of my earlier copending application, Ser. No. 76,789, filed Sept. 30, 1970 now abandoned. A claim for priority based on a corresponding German application was made and perfected in said earlier application.

The present invention relates to a thermal engine operating according to the Stirling process, especially a piston engine, which includes a power engine (prime mover) hereafter referred to as a hot gas section and an output or work engine, hereafter referred to as a cold gas section between which a gaseous working medium is interchangeable by way of a regenerator.

With prior art thermal engines of this type, the hot gas section and cold gas section are connected by way of a regenerator, whereby a heater is connected to the outlet side of the regenerator in the transition from the hot section to the cold section in which the working medium flowingto the hot section or prime mover is heated indirectly, i.e., by heat conduction and heat radiation. By the use of the best of the available, thermally highly resistant materials for the heat-exchanger, temperatures of about l,200C. for the working medium may be obtained maximum. A further heating-up of the working medium is desirable as regards the thermal efficiency of the engine and is also theoretically possible, but fails on the basis of the thermal loading capacity of the materials used for the heat-exchanger which materials fail with the increased heating of the heat-exchanger required for a further increase of the end temperature of the working medium.

With a thermal engine of the aforementioned type, the above discussed disadvantage is avoided according to the present invention by a construction, in which the working medium is heated directly on the hot gas section side by supplying and combustion of fuel, and in which the used-up working medium is replaced at least partially by fresh working medium on the cold gas section side. At the outset, those limitations resulting from the use of a heat exchanger in the heating-up of the working medium are eliminated by the direct heating of the working medium according tothe present invention. The solution according to the present invention is also advantageous in that the structural volume of the engine is considerably decreased as a result of the elimination of the heat-exchanger. Also, this arrangement considerably reduces cost of the engine.

In actual construction of the engine, it has proved advantageous to undertake the heating of the working medium directly in the hot gas section. Separate combustion chambers are avoided thereby. Furthermore, it is also appropriate to exchange the working medium in the cold gas section of the engine.

A constructively particularly simple assembly of such a thermal engine can be obtained in that both the cold gas section and the hot gas section are constructed as piston engines, and more particularly as reciprocating piston engines, in which the piston of the cold gas section and the piston of the hot gas section are positively controlled with appropriate phase displacementtherebetween.

According to a further feature of the present invention, at least one cold gas section and one hot gas section, which are constructed as reciprocating piston engines, may be disposed mutually opposite one another forming an engine unit and may be rigidly connected with each other by way of the regenerator as intermediate connecting member whereby the positive coupling necessary for the phase-displaced drive of the pistons in the two spaces can be achieved in a simple manner. Moreover, with this basic type of arrangement a very simple constructive assembly may also be attained altogether, in which recourse may be had particularly farreachingly to known and tested structural elements.

The feed of the fuel into the power engine takes place according to the present invention in an advantageous manner by injection.

The control of the at least partial exchange of the working medium displaced out of the hot gas section by way of the regenerator into the cold gas section may take place according to the present invention by way of slots so that, especially with the use of a reciprocating piston engine, particularly simple and also easily controllable gas-exchange operations result.

An advantageous feature of the present invention is the arrangement of the control of the partial exchange of the working medium such that the exchange starts during the late stages of the expansion ofthe cold gas section and continues into the early stages of the compression of the cold gas section. In this manner the amount of total engine work lost in the cold gas section is minimized. This particular timing of the medium exchange provides a substantial improvement in the efficiency of the operation of the cold gas section as compared to an arrangement introducing fresh medium at an earlier stage of the expansion phase of the cold gas section as exemplified by US. Pat. No. l55,087 to l-lirsch in relation to a hot air engine.

According to a still further feature of the present invention the installation for the exchange of the working medium associated with the cold gas section may include a supercharger, by means of which fresh working medium is adapted to be supplied precompressed to the cold gas section whereby appropriately the supercharging takes place by means of an exhaust-gas turbocharger. With this supercharger arrangement flushing or purging of the working medium is achieved and a p'recompression of the workingmedium is obtained.

An alternative arrangement of the present invention may utilize engine crankcase vacuum pressure to assist in the medium exchange.

The advantageous arrangements of the present invention may be utilized with both axially reciprocating piston engines and rotary piston engines.

By the construction according to the present invention of a thermal engine of the aforementioned type, a high specific output thereof may be attained and in particular also an improvement of the thermal efficiency and therewith of the energy utilization can be achieved due to the utilization of the energy in the cold gas section.

Accordingly, it is an object of the present invention to provide a thermal engine of the type described above which avoids by simple means the aforementioned shortcomings and drawbacks encountered in theprior art.

Another object of the present invention resides in a thermal engine which permits a further heating-up of the working medium without the need of costly and complicated heat-exchangers.

A further object of the present invention resides in a thermal engine in which not only the pre-existing limi- 3 tations as regards the heating-up of the working medium are eliminated but which also excels by a small structural volume of the engine, thereby reducing its weight and cost. 1

Another object of the present invention resides in a thermal engine of the type described above which farreachingly utilize known, tested structural elements to assure reliable operation by relatively inexpensive means.

These and further objects, features and advantages of the present invention will become more obvious from the following description when taken in connection with the accompanying drawing which shows, for purposes of illustration only, two embodiments in accordance with the present invention, and wherein:

FIG. 1 is a schematic view illustrating a first embodiment of an engine according to the present invention which includes axially reciprocating pistons in each of the hot and cold gas sections;

FIG. 2 is a schematic view taken perpendicularly to the view of FIG. 1;

FIG. 3 is a schematic view illustrating a second embodiment of an engine according to the present invention which includes a rotary piston;

FIG. 4 is a schematic view taken perpendicularly to the view of FIG. 3; and

FIG. 5 is a pressure-volume diagram comparing the efficiency of a cold gas section having a medium exchange in accordance with the present invention with a cold gas section having a medium exchange throughout the expansion of the cold gas section.

Referring now to FIGS. 1 and 2 of the drawing, reference numerals 1 and 2 designate respective hot and cold gas sections arranged mutually opposite one another on opposite sides of a regenerator 3. These sections 1 and 2 include piston and cylinder arrangements which are connected at the cylinder head side by way of the conventional regenerator 3 which is permeable to the working medium, i.e., permits its passage therethrough. Of the two sections 1 and 2, the hot gas section 1 operates as prime mover or power engine whereas the cold gas section 2 operates as work or output engine.

The cylinder 8 of the cold gas section 2 is provided in the illustrated embodiment with at least one inlet slot 4 and with at least one outlet slot 5, by way of which the exchange of the working medium takes place. Slots 4 and 5 are arranged in cylinder 8 so as to be open when the piston 6 is near the bottom of the cylinder. Whereby the exchange of the working medium takes place during the transition from the expansion to the compression condition of the cold gas section. By introducing the fresh medium only after a substantial portion of the expansion takes place, the efficiency of the cold gas section is maximized.

The hot gas section 1, which is constructed essentilly similarly to the cold gas section described, includes a piston 6', a crank drive 7', and a cylinder 8'; however, in contradistinction to the cylinder 8 of the cold gas section 2, the cylinder 8 of the hot gas section 1 is closed and does not include any control channels or slots. The positive connection between the crank drives 7 and 7' of the cold gas section 2 and the hot gas section 1, by means of which the pistons 6 and 6 are positively controlled with phase displacement includes a gear wheel arrangement 11 with gear wheels 12 and 12' operatively connected at the respective crank drives 7 and 7. These gear wheels 12 and 12' are in driving connection with each other by four connecting gears 13.

A fuel injection installation 9 is provided for introducing fuel to be combusted in the cylinder 8' of the hot gas section 1. A pump 16 of this installation 9 is drivingly connected with one of the connecting gears 13 by way of cogwheel l4 and shaft 15.

The operational cycle of the installation shown in FIGS. 1 and 2 is as follows. The working medium in the cold gas section 2 is pushed by piston 6 through regenerator 3 into the hot gas section 1. The working medium is heated by the regenerator due to the heat absorbed by the regenerator during the previous cycle. This heated working medium is then further heated by the combustion of fuel injected by installation 9 into the hot gas section 1. This further heating by combustion of the fuel limits the pressure drop of the working medium during expansion of the working medium in cylinder 8', which expansion provides the force for raising piston 6'. After the piston 6 has passed the top dead center position corresponding to maximum expansion in the hot gas section, the working medium is pushed back through the regenerator 3, into the cold gas section 1. During passage of the working medium from the hot gas section tothe cold gas section, the regenerator 3 absorbs heat from the working medium. As the working medium builds up in the cylinder 8 of the cold gas section 2, the piston 6 is moved downwardly to expand the cold gas section. As the piston 6 approaches its bottom dead center position corresponding to maximum expansion of the cold gas section, inlet slot 4 and outlet slot 5 are opened to the top piston 6 to effect a partial exchange of the working medium. It is noted that a partial exchange of the working medium is necessary to replenish oxygen necessary for the combustion of the fuel which takes place in the hot gas section. By arranging the slots 4 and 5 so that the partial exchange only takes place after completion of a substantial portion of the expansion in the cold gas section, the efficiency of the cold gas section is substantially improved as compared to an arrangement for initiating the exchange earlier in the expansion stroke (see FIG. 5 and related description). After completion of the expansion stroke of piston 6, the working medium as pushed back through the regenerator and the above described steps are continuously repeated.

It is to be understood that the inside of regenerator 3 will experience a temperature gradient due to the much stronger heating at the hot gas section side thereof.

FIG. 5 graphically compares the efficiency of a cold gas section operating according to the present invention with initiation of the partial exchange of medium only after a substantial portion of the expansion in the cold gas section has taken place with the efficiency of a cold gas section having the partial exchange taking place throughout the expansion in the cold gas section. In FIG. 5 the area enclosed by the upper and lower curved lines represents the negative work of the cold gas section of an experimental engine operating according to the present invention. The area enclosed by the upper curved line and the horizontal line at 2 atmosphere represents the negative work of a cold gas section wherein fresh medium is introduced throughout the expansion phase, with an assumed minimum pressure of 2 atmospheres, the same as in the experimental engine. The area between the lower curved line and the horizontal 2 atmosphere line represents the difference in negative work between the two cold gas sections. Under the assumption that the hot gas sections of the two engines operate with comparable efficiencies, this difference in negative work in the cold gas section represents the total difference in efficiency of the two engines.

in a preferred embodiment of the present invention, approximately 70 percent to 75 percent of the expansion takes place prior to opening for the partial exchange. As can be seen from FIG. 5, this arrangement practically optimizes the utilization of available expansion energy. The upper edges of the outlet slots are disposed at a spacing from the bottom dead center position a distance of approximately 25 percent to 30 percent of the piston travel path.

In the embodiment according to FIGS. 1 and 2, the coaxial alignment of the cylinders further improves the total efficiency by minimizing the dead space area that would be present if additional ducting were utilized.

in order to improve the specific output of the engine, the energy of the exhaust gases flowing out by way of the exhaust channel 5 may be utilized in a known manner for the drive of a supercharger, for example, of a turbo-charger 10, by means of which working medium under superor excess-pressure is fed to the cold gas section 2 by way of the inlet channel 4. This turbocharger betterv enhances the purging of cold gas medium and precompresses the fresh medium.

It is merely schematically indicated in the illustrated embodiments that the fuel for the combustion is fed or supplied to the power engine 1, preferably is injected for installation. However, not shown in the drawing is the fact that, in order to maintain the thermal load of the power engine 1 within the area of the combustion zone within tolerable limits, a special combustion chamber may also be provided which may be provided, for example, in the transition between the regenerator 3 and the power engine 1.

By the direct heating of the working medium according to the present invention, temperatures are attainable for the working medium, which'lie considerably higher than with the heretofore customary constructions, in which the working medium was heated up only indirectly. Furthermore, a considerably more compact construction is attainable thereby since the installations necessary for the indirect heating can be dispensed with. The heating-up to consierably higher temperatures, on the one hand, and the elimination especially of bulky structural parts, on the other, produces a particularly favorable weight-to-output ratio of the thermal engine according to the present invention. This can be still further improved by the supercharger installation provided according to the present invention. Therebeyond, the construction according to the embodiments of H65. 1 and 2 of the'present invention excels also by an altogether particularly simple structure because the oppositely directed movement of the pistons can be controlled phase-directed in a simple manner and separate housing and ducting to and from the regenerator can be dispensed with.

P105. 3 and 4 iltustrate a rotary piston engine embodiment of the present invention which utilized the same basic advantageous features described above for the axially reciprocating piston embodiment of FIGS. 1 and 2.

This engine has a housig 100 including a housing jacket 101 with laterally extending housing parts 102 and 103 at opposite sides thereof. The jacket 101 is provided with a four-arc track 104. Within the housing, a rotary piston 1115 is disposed in an eccentrically rotatable manner. This piston 105 is of a three arc structure with a trochoidal shape.

The engine comprises four working chambers 106, 106', 107, 107', of which two in each case form a working unit, namely 1116 and 1117 and 106' and 107, respectively. These chambers are in communication by way of respective regenerator 108 and 108. The regenerator is connected to the associated chambers by ducts 109, 110 and 109, 110', respectively.

In the lateral housing parts or portions 102 and 103, gas exchange channels 111, 112, and 111', 112', respectively, are provided whichterminate in respective chambers 106, 106. The apertures of these channels which terminate in the side walls arecontrolled by piston 105 such that a slot control is provided forthe gas or working medium exchange. The respective gas exchange channels, forming inlet channels 111, 111 and outlet channels 112, 112', are in communication with respective superchargers 113 and 113'.

In operation, the chambers 106 and 106. pertain to the cold gas section while chambers 107 and 107', pertain to the hot gas section of the engine. Fresh working medium enters chambers 106 or 106 through channels 111 or 111', respectively. The working medium is then compressed and transferred by way of regenerator 108 or 108 into the respective chamber 107 or 107 associated withthe hot gas section of the engine. Then fuel is injected by nozzles 114' or 114' into chamber 107 or 107' where it is combusted to further heat the medium. An injection pump 115, common to both nozzles 114 and 114 and driven by shaft 116 of the engine through a belt drive or the like, effects the introduction of fuel to the nozzles. I

After this combustion heating of the working medium, the medium is expanded in the chamber 107 or 107 and is then pushed back through regenerator 108 or 108' into chamber 106 or 106' where cold gas section expansion takes place. Toward the end of this cold gas .section expansion, the apertures of the gas exchange channels 111, 112 are uncovered by the piston to permit the partial exchange of the working or operating medium.

The above described process is then repeated continuously for two working units 106, 107 and 106, 107 of the engine.

The thermal engines according to the present invention combines the advantages of the known combustion engines with internal combustion; namely, high-process temperature and high output concentration, as well as those of the hot gas engine; namely, an efficiency approximating the Carnot-process, a smooth running and suitability for multi-fuel operation. Furthermore, the engine according to the present invention can be constructed of light whight since the pressure level is relatively low and the pressure rise is relatively small, and this is the case notwithstanding a large effective average pressure.

While 1 have shown and described only two embodiments in accordance with the present invention, it is understood that the same is not limited thereto but is susceptible of numerous changes and modifications as known to those skilledin the art, and I therefore do not wish to be limited to the details shown and described herein but intend to cover all such changes and modifications as are encompassed by the scope of the appended claims.

I claim:

1. A thermal engine operating according to the Stirling process comprising: at least one hot gas section, at least one cold gas section, a gaseous working medium interchangeable between said sections by way of a regenerator means, means for directly heating the working medium on the side of the hot gas section by a supply and combustion of fuel, and means for replacing on the side of the cold gas section at least partially the combusted working medium by fresh working medium, the means for the exchange of the working medium associated with the cold gas section including supercharger means for precompressing the fresh working medium to be supplied to the cold gas section, an exhaust gas turbocharger menas being provided for supercharging the fresh working medium.

2. A thermal engine operating according to the Stirling process comprising: hot gas section means having a first variable volume working chamber, cold gas section means having a second variable volume working chamber, a gaseous working medium, regenerator means for communicating the working medium between the first and second chambers, said cold gas section means including means for reducing the volume of said second chamber to compress the working medium prior to the passage of said working medium from the second chamber to said first chamber by way of the regenerator means, combustion heating means for directly heating said medium while said medium is in said first chamber, said heating means including fuel supply means and fuel combustion means, said hot gas section means including means for forcing the heated medium back through the regenerator means, said regenerator means absorbing heat from the medium as itpasses therethrough, and medium exchange means arranged in communication with the second chamber for effecting at least a partial exchange of the working medium that has been heated by the heating means and passed back through the regenerator means, saidmedium exchange means including means for replacing at least part of said working medium with fresh working medium and control means for effecting said partial exchange during the transition of said second chamber from the expansion condition to the compression condition, said control means including means for keeping said second chamber closed until near the end of the expansion condition so as to delay said at least partial exchange, said expansion condition occurring after the medium passes from the regenerator into the second chamber and said compression condition occurring before the medium is passed back through the regenerator for the next cycle.

3. A thermal engine according to claim 2, characterized in that both the hot gas section means and the cold gas section means include reciprocating pistons, said pistons being slidably disposed in cylinder means which delimit the respective first and second working chambers.

4. A thermal engine according to claim 2, characterized in that positive coupling means are provided for coupling the hot and cold gas section means together with predetermined phase displacement.

5. A thermal engine according to claim 3, characterized in that the cylinder means of the hot gas section means is axially aligned with the cylinder means of the cold gas section means, the pistons of the hot and cold gas section means being arranged in said cylinder means mutually opposite one another with said regenerator means arranged between the ends of said pistons, said regenerator means being rigidly connected to said cylinder means to form a single engine unit with the respective hot and cold gas section means.

6. A thermal engine according to claim 5, characterized in that said control means includes slot means provided on the cylinder means associated with tbe cold gas section means, said slot means being arranged to permit-entry and exhaust of working medium when the associated piston is in a predetermined position.

7. A thermal engine according to claim 6, characterized in that said slot means are arranged to be open only when said piston is near the dead center bottom position corresponding to the transition from expansion to compression of the second chamber.

8. A thermal engine according to claim 3, characterized in that said control means includes slot means provided on the cylinder means associated with the cold gas section means, said slot means being arranged to permit entry and exhaust of working medium when the associated piston is in a predetermined position.

9. A thermal engine according to claim 8, characterized in that said slot means are arranged to be open only when said piston is near the dead center bottom position corresponding to the transition from expansion to compression of the second chamber.

10. A thermal engine according to claim 6, characterized in that the means for the exchange of the working medium associated with the cold gas section means includes supercharger means for precompressing the fresh working medium to be supplied to the cold gas section means.

11. A thermal engine according to claim 8, characterized in that the means for the exchange of the working medium associated with the cold gas section means includes supercharger means for precompressing the freshworking medium to be supplied to the cold gas section means.

12. A thermal engine according to claim 2, characterized in that said hot gas'section means includes a power piston displaceably slidable in a first cylinder, said first chamber being formed by the end of said power piston and the walls of the first cylinder, said cold gas section means including an output piston displaceably slidable in a second cylinder, said second chamber being formed by the end of said output piston and the walls of the second cylinder, said first and second cylinders being axially aligned with the first and second chambers facing one another, said regenerator being positioned between said first and second chambers and being rigidly connected to said first and second cylinders.

13. A thermal engine according to claim 2, characterized in that said hot and cold gas section means are included as parts of a rotary piston engine.

14. A thermal engine according to claim 4, characterized in that said hot and cold gas section means are included as parts of a rotary piston engine.

15. A thermal engine according to claim 14, characterized in that the means for the exchange of the working medium associated with the cold gas section means includes Supercharger means for precompressing'the fresh working medium to be supplied to the output engine.

16. A thermal engine according to claim 15, characterized in that an exhaust gas turbo-charger means is provided for supercharging the fresh working medium.

17. A thermal engine according to claim 13, characterized in that said second chamber is formed in an engine housing with side walls delimiting the lateral edges of said second chamber, in that a rotating rotary piston effectively varies the volume of said second chamber, and in that said control means includes apertures in said side walls which are opened and closed with respect to said second chamber by said piston.

18. A thermal engine according to claim 17, characterized in that said engine comprises four spaced working chambers formed by said side walls and a circumferentially four-arc track, two of said working chambers forming respective hot and cold gas sections in communication with one another by a regenerator unit, the other two of said working chambers forming another respective hot and cold gas section in communication with one another by another regenerator unit,

and in that said rotary piston is of three-arc structure with a trochoidal shape.

19. A thermal engine according to claim 17, characterized in that said apertures are so positioned with respect to said second chamber and said piston path that medium exchange is permitted only after a substantial portion of the expansion in the second chamber is completed.

20. A thermal engine according to claim 2, characterized in that the means for the exchange of the working medium associated with the cold gas section means includes supercharger means for precompressing the fresh working medium to be supplied to the cold gas section means.

21. A thermal engine according to claim 20, characterized in that the supercharger means is constructed as an exhaust gas turbo-charger means.

22. A thermal engine according to claim 21, characterized by positive coupling means for coupling the two gas sections together with predetermined phase displacement. 

1. A thermal engine operating according to the Stirling process comprising: at least one hot gas section, at least one cold gas section, a gaseous working medium interchangeable between said sections by way of a regenerator means, means for directly heating the working medium on the side of the hot gas section by a supply and combustion of fuel, and means for replacing on the side of the cold gas section at least partially the combusted working medium by fresh working medium, the means for the exchange of the working medium associated with the cold gas section including supercharger means for precompressing the fresh working medium to be supplied to the cold gas section, an exhaust gas turbocharger menas being provided for supercharging the fresh working medium.
 2. A thermal engine operating according to the Stirling process comprising: hot gas section means having a first variable volume working chamber, cold gas section means having a second variable volume working chamber, a gaseous working medium, regenerator means for communicating the working medium between the first and second chambers, said cold gas section means including means for reducing the volume of said second chamber to compress the working medium prior to the passage of said working medium from the second chamber to said first chamber by way of the regenerator means, combustion heating means fOr directly heating said medium while said medium is in said first chamber, said heating means including fuel supply means and fuel combustion means, said hot gas section means including means for forcing the heated medium back through the regenerator means, said regenerator means absorbing heat from the medium as it passes therethrough, and medium exchange means arranged in communication with the second chamber for effecting at least a partial exchange of the working medium that has been heated by the heating means and passed back through the regenerator means, said medium exchange means including means for replacing at least part of said working medium with fresh working medium and control means for effecting said partial exchange during the transition of said second chamber from the expansion condition to the compression condition, said control means including means for keeping said second chamber closed until near the end of the expansion condition so as to delay said at least partial exchange, said expansion condition occurring after the medium passes from the regenerator into the second chamber and said compression condition occurring before the medium is passed back through the regenerator for the next cycle.
 3. A thermal engine according to claim 2, characterized in that both the hot gas section means and the cold gas section means include reciprocating pistons, said pistons being slidably disposed in cylinder means which delimit the respective first and second working chambers.
 4. A thermal engine according to claim 2, characterized in that positive coupling means are provided for coupling the hot and cold gas section means together with predetermined phase displacement.
 5. A thermal engine according to claim 3, characterized in that the cylinder means of the hot gas section means is axially aligned with the cylinder means of the cold gas section means, the pistons of the hot and cold gas section means being arranged in said cylinder means mutually opposite one another with said regenerator means arranged between the ends of said pistons, said regenerator means being rigidly connected to said cylinder means to form a single engine unit with the respective hot and cold gas section means.
 6. A thermal engine according to claim 5, characterized in that said control means includes slot means provided on the cylinder means associated with tbe cold gas section means, said slot means being arranged to permit entry and exhaust of working medium when the associated piston is in a predetermined position.
 7. A thermal engine according to claim 6, characterized in that said slot means are arranged to be open only when said piston is near the dead center bottom position corresponding to the transition from expansion to compression of the second chamber.
 8. A thermal engine according to claim 3, characterized in that said control means includes slot means provided on the cylinder means associated with the cold gas section means, said slot means being arranged to permit entry and exhaust of working medium when the associated piston is in a predetermined position.
 9. A thermal engine according to claim 8, characterized in that said slot means are arranged to be open only when said piston is near the dead center bottom position corresponding to the transition from expansion to compression of the second chamber.
 10. A thermal engine according to claim 6, characterized in that the means for the exchange of the working medium associated with the cold gas section means includes supercharger means for precompressing the fresh working medium to be supplied to the cold gas section means.
 11. A thermal engine according to claim 8, characterized in that the means for the exchange of the working medium associated with the cold gas section means includes supercharger means for precompressing the fresh working medium to be supplied to the cold gas section means.
 12. A thermal engine according to claim 2, characterized in that said hot gas section means includes a pOwer piston displaceably slidable in a first cylinder, said first chamber being formed by the end of said power piston and the walls of the first cylinder, said cold gas section means including an output piston displaceably slidable in a second cylinder, said second chamber being formed by the end of said output piston and the walls of the second cylinder, said first and second cylinders being axially aligned with the first and second chambers facing one another, said regenerator being positioned between said first and second chambers and being rigidly connected to said first and second cylinders.
 13. A thermal engine according to claim 2, characterized in that said hot and cold gas section means are included as parts of a rotary piston engine.
 14. A thermal engine according to claim 4, characterized in that said hot and cold gas section means are included as parts of a rotary piston engine.
 15. A thermal engine according to claim 14, characterized in that the means for the exchange of the working medium associated with the cold gas section means includes supercharger means for precompressing the fresh working medium to be supplied to the output engine.
 16. A thermal engine according to claim 15, characterized in that an exhaust gas turbo-charger means is provided for supercharging the fresh working medium.
 17. A thermal engine according to claim 13, characterized in that said second chamber is formed in an engine housing with side walls delimiting the lateral edges of said second chamber, in that a rotating rotary piston effectively varies the volume of said second chamber, and in that said control means includes apertures in said side walls which are opened and closed with respect to said second chamber by said piston.
 18. A thermal engine according to claim 17, characterized in that said engine comprises four spaced working chambers formed by said side walls and a circumferentially four-arc track, two of said working chambers forming respective hot and cold gas sections in communication with one another by a regenerator unit, the other two of said working chambers forming another respective hot and cold gas section in communication with one another by another regenerator unit, and in that said rotary piston is of three-arc structure with a trochoidal shape.
 19. A thermal engine according to claim 17, characterized in that said apertures are so positioned with respect to said second chamber and said piston path that medium exchange is permitted only after a substantial portion of the expansion in the second chamber is completed.
 20. A thermal engine according to claim 2, characterized in that the means for the exchange of the working medium associated with the cold gas section means includes supercharger means for precompressing the fresh working medium to be supplied to the cold gas section means.
 21. A thermal engine according to claim 20, characterized in that the supercharger means is constructed as an exhaust gas turbo-charger means.
 22. A thermal engine according to claim 21, characterized by positive coupling means for coupling the two gas sections together with predetermined phase displacement. 